Ballistic tolerant dual load path ballscrew and ballscrew actuator

ABSTRACT

Ballscrew actuators are well known for use as a primary flight control actuator for operating a flight control surface. The operation of the ballscrew actuator is critical to the success of an aircraft mission; however, when the actuator is subject to ballistic fire, a projectile hit which is not totally destructive can still render the actuator inoperable. Increasing the potential for operability of the ballscrew actuator after a projectile hit which is not totally destructive will increase the number of successful returns from aircraft missions. 
     The ballistic tolerant dual load path ballscrew actuator has a selectively-driven ballnut and a ballscrew drivingly connected to the ballnut. The ballscrew has an outer tubular member with an external helical ball groove providing a primary load path. An inner tubular member fits closely within and is fixed to the outer tubular member to provide a secondary load path upon cracking of the outer tubular member by a projectile passing through the outer tubular member. The inner tubular member is formed of ballistic tolerant composite material to maintain its structural integrity when contacted by a projectile which has passed through and cause cracking of the outer tubular member. In one embodiment, the inner tubular member has an outer energy absorbing layer of a ceramic material. In another embodiment, the inner tubular member is preloaded in tension to prevent propagation of a circumferential crack in the outer tubular member and is constructed of wound fibers providing ballistic tolerance and strength in torsion and tension.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.686,983, filed Dec. 27, 1984.

DESCRIPTION

1. Field of the Invention

This invention relates to a ballistic tolerant dual load path ballscrewand, more particularly, a ballscrew actuator having either a singleballscrew or telescoped ballscrews. Each ballscrew has an outer tubularmember formed with a helical groove for coaction with a ballnutproviding a primary load path and an inner tubular member operable toprovide a secondary load path in the event of a failure of the outertubular member. The inner tubular member is formed of ballistic tolerantmaterial whereby a ballistic hit reaching the inner tubular member,after passing through and cracking the outer tubular member, will notcause the inner tubular member to lose its structural integrity and theload-carrying capability of the ballscrew is maintained. The tubularmember can be pre-loaded in tension, and the inner tubular memberspecially constructed for such purpose, to prevent separation of acircumferentially-cracked outer tubular member.

2. Background of the Invention

There are many components of an aircraft that are positioned byoperation of a ballscrew actuator. In such an actuator, there isrelative rotation between a ballscrew and a ballnut to achieve lineartranslation of one of these elements. One of the actuator elements isfixed and the other actuator element is connected to a flight controlsurface or other component to be moved. Because of use of such aballscrew actuator in controlling critical surfaces for aircraft flight,it is common to construct the ballscrew of inner and outer tubularmembers which are interconnected at their ends to provide a dual loadpath. The outer tubular member provides a primary load path for theballscrew and the inner tubular member provides a secondary load path tomaintain the integrity of the ballscrew. This structure is known as adual load path ballscrew.

The outer tubular member has a helical ball groove which is associatedwith the ballnut by means of balls positioned in a portion of thehelical groove. As an example, rotation of the ballnut will cause lineartranslation of the ballscrew for transmission of force and positioningof an actuated surface.

The dual load path ballscrew actuator is sometimes used as a primaryflight control actuator in aircraft which are subject to ballistic fire.It is critical to the success of the aircraft mission that the ballscrewactuator continue to function after a ballistic hit which has damaged,but not destroyed, the ballscrew actuator. Upon the occurrence of adamaging ballistic hit, the ballscrew must maintain structural integrityand must be failsafe. The outer tubular member is commonly made of ahardened steel, and a ballistic hit on this member can cause it to crackin a way that results in failure. This result could also occur when theprojectile reaches the inner tubular member of conventional metalconstruction.

The invention disclosed herein provides for improvement over the priorart in having the inner tubular member formed of ballistic-tolerantmaterial which will not substantially sacrifice structural integritywhen hit with a ballistic projectile. The ballistic-tolerant materialserves to preserve the secondary load path established by the innertubular member and adds to the energy absorption capability of theballscrew. Additionally, the inner tubular member may have an energyabsorbing outer layer and be preloaded in tension to prevent separationof a cracked outer tubular member.

SUMMARY OF THE INVENTION

A primary feature of the invention is to provide a ballistic-tolerantdual load path ballscrew actuator.

A ballscrew has an outer tubular member with a helical groove and ismade of a hardened material which is suitable to withstand high contactloads during operation. This requirement means that the outer tube issubject to crack growth and subsequent failure as a result of aballistic hit. The inner member is positioned within and fixed to theouter tubular member and is formed of a ballistic tolerant compositematerial which will maintain its structural integrity when hit by aprojectile which has passed through and possibly cracked the outertubular member to maintain the load-carrying capability and possibleoperability of the ballscrew.

The foregoing is achieved by having the inner member as a tubular memberclosely fitted within the outer tubular member and secured thereto andwith the composite material being of a resin-bonded cross-woven fiber.The composite material is of a thickness to either permit passage of aprojectile therethrough or to retain the projectile against passagetherethrough with deformation of the inner tubular member and with thehandling characteristic of the projectile being dependent upon the fiberand weaving characteristics thereof as well as the thickness of thecomposite material.

The ballscrew actuator may have either a single ballscrew, as defined inthe preceding paragraph, or can have a pair of the ballscrews ofdifferent diameters and telescopically related, whereby there are innerand outer ballscrews and the inner ballscrew lies substantially withinthe outer ballscrew when the actuator is retracted. The characteristicsof the inner and outer ballscrews can be varied respecting the responseto a ballistic hit by selection of the composite material forming theinner tubular member of the inner and outer ballscrews. The innertubular members may be constructed to permit passage of a projectiletherethrough or "catching" of the projectile with deformation of theinner tubular member.

Also, there is the possibility of constructing the inner tubular memberof the outer ballscrew to "catch" a projectile and the inner tubularmember of the inner ballscrew being constructed of a composite materialto either permit a projectile to pass therethrough when it is an"aligned" round and to catch a "tumbled" round. In this configuration,the outer ballscrew serves as protection for the inner ballscrew whenthe actuator is retracted. This prevents the potential of a jam due to alodged projectile unable to penetrate the eight surfaces of the twoballscrews and allows the inner ballscrew to operate. The outerballscrew will still have the capability to hold a load, but is unlikelyto translate through the associated ballnut and, thus, cannot operate.

Additionally, the inner tubular member of the ballscrew can have aceramic shell which is bonded to, or co-cured with, the outer surface ofthe inner tubular member to blunt or shatter a projectile prior to itsreaching the composite material to facilitate use thereof as a catcher'smitt to catch the projectile.

When there is a ballistic hit on the ballscrew actuator, there canresultingly be a circumferential crack in the outer tubular member. Theinner tubular member is connected at its ends to the outer tubularmember and can be pre-loaded in tension to prevent axial separation ofthe outer tubular member which precludes jamming of a ball of theballnut in the cracked outer tubular member. An enhancement of thisstructure resides in forming the inner tubular member of wound materialwhich is wound at two different angles.

An object of the invention is to provide a ballistic tolerant, dual loadpath ballscrew actuator having a selectively driven ballnut and aballscrew operatively connected to said ballnut, said ballscrew havingan outer tubular member with an external helical ball groove providing aprimary load path and an inner tubular member closely fitting within andfixed to the outer tubular member to provide a secondary load path uponcracking of the outer tubular member by a projectile passing through theouter tubular member, said inner tubular member being formed ofballistic tolerant composite material to maintain its structuralintegrity when contacted by said projectile which has passed through andcaused cracking of the outer tubular member.

Still another object of the invention is to provide a ballistictolerant, dual load path ballscrew actuator as defined in the precedingparagraph wherein the ballistic tolerant material is a composite ofmaterials which can be selected to either permit a projectile to pierceand pass through the inner tubular member or to deform upon being hit bya projectile without piercing thereof by the projectile.

Another object of the invention is to provide a ballistic tolerant dualload path ballscrew actuator wherein said composite material is across-woven aramid fiber in a resin binder to achieve a structure whichdeforms, but is not pierced by a projectile.

Still another object of the invention is to provide a ballistictolerant, dual load path ballscrew actuator wherein the compositematerial is resin-bonded cross-woven fiber glass.

A further object of the invention is to provide a ballistic tolerant,dual load path ballscrew actuator wherein the composite material (glassor aramid fiber in a resin matrix) is covered with a layer of ceramic(partially stabilized zirconia [PSZ]) to blunt or shatter a projectileprior to its being caught by the composite material.

Still another object of the invention is to provide a ballistictolerant, dual load path ballscrew actuator as defined in the precedingparagraphs wherein the inner and outer tubular members are bondedtogether along the entire length of the inner tubular member to aid inenergy absorption and further optionally wherein the tubular members aremechanically interconnected.

Still another object of the invention is to provide a ballscrew usablein a ballscrew actuator and having the characteristics set forth in thepreceding paragraphs.

A ballscrew manufactured in accordance with the teachings of thisinvention can be adapted to meet many differing requirements inrendering the ballscrew actuator ballistically tolerant. The innertubular member providing the secondary load path can be designed to meetseveral different requirements including the nature of the ballisticthreat and, particularly, the size and energy content of the projectileto which the ballscrew actuator may be subjected as well as the requireddesign wall thickness of the outer tubular member normally formed ofsteel. The inner tubular member can be constructed of a ballistictolerant composite material to either permit a projectile to passtherethrough, act as a "catcher's mitt" to stop the projectile, orpermit passage of the projectile therethrough when the projectile is an"aligned" round and catch the projectile when the projectile is a"tumbled" round. In all of these cases, the inner tubular memberperforms its ballistic tolerant function without sacrificing theload-carrying capability thereof and, in cases of minor damage,maintains the outer tubular member in coacting relation with anassociated ballnut.

A composite material which will permit the projectile to pass throughthe inner tubular member can be a resin-bonded cross-weave fibercomposite which permits a projectile to pass through it, like pushing apencil through a screen door. Some of the fibers may be damaged, butcrack propagation is minimized. An inner tubular member catching aprojectile can be formed of a resin-bonded fiber composite using a fibersuch as an aramid fiber having a very high strength-to-weight ratio.With a sufficient thickness, this material will not permit a projectileto pass therethrough, but will deform and act as an energy absorber. Analternate structure as immediately above described has the wall of theinner tubular member thinner and, thus, the energy of an "aligned" roundwill result in the projectile passing therethrough while the lesserenergy in a "tumbled" round will merely cause deformation of the innertubular member. The cross-weaving of the fibers may result in some fiberdamage, but crack propagation will not occur.

Further enhancement of the operability of the ballistic tolerantballscrew resides in a tight fit of the ballscrew and inner tube whichaids in energy absorption with means to prevent axial separation of thescrew. One method is by bonding of the inner and outer tubular membersto each other with an adhesive and with a mechanical connection of themembers as a back-up to the adhesive bonding. This assures transfer oftorsional loads between members. There can also be a mechanicalconnection under tension without bonding to prevent axial separation.This enables the balls to roll on the threads with a circumferentialcrack.

Another object of the invention is to provide a ballistic tolerant, dualload path ballscrew having interconnected inner and outer tubularmembers with the inner tubular member pre-loaded in tension to preventseparation of a circumferentially-cracked outer tubular member.

A further object of the invention is to provide a ballscrew as definedin the preceding paragraph wherein the inner tubular member is formed ofa composite material having two sets of windings therein, with one setwound at a lesser angle than the other to act in tension under saidpre-loading.

An additional enhancement to the capability of the structure towithstand ballistic projectiles is provided by bonding or co-curing aceramic shell to the outer surface of the composite inner tubularmember. The ceramic will serve to blunt or shatter a projectile and willallow the composite to more easily act as a "catcher's mitt".

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of the ballistic tolerant dual loadpath ballscrew actuator with the upper half thereof shown in verticalsection;

FIG. 2 is a view, similar to FIG. 1, of an alternate embodiment of theballscrew actuator having inner and outer telescoping ballscrews;

FIG. 3 is a fragmentary view of a ballscrew as used in the ballscrewactuator and showing the composite material of the inner tubular member;

FIG. 4 is a diagrammatic view illustrating the action of a projectilewith respect to one ballscrew construction;

FIG. 5 is a diagrammatic view illustrating the action of a projectilewith respect to another construction of ballscrew;

FIG. 6 is a diagrammatic view of a telescoping ballscrew actuatorshowing different projectile actions dependent upon different structuresof the ballscrews;

FIG. 7 is a view, similar to FIG. 3 and on a reduced scale, of anotherembodiment of an inner tubular member;

FIG. 8 is a fragmentary longitudinal section of a further embodiment ofthe invention;

FIG. 9 is an enlarged view of a portion of the right-hand end of FIG. 8;

FIG. 10 is an enlarged view of a portion of the left-hand end of FIG. 8;

FIG. 11 is a sectional view, taken generally along the line 11--11 inFIG. 9; and

FIG. 12 is a fragmentary view, taken generally along the line 12--12 inFIG. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the invention is shown in FIG. 1 wherein aballscrew, indicated generally at 10, is associated with a ballnut,indicated generally at 12. The ballnut 12 is rotatably mounted within ahousing, indicated generally at 14, and the ballscrew 10 is fixed at 16to a connecting member 18. The housing 14 can be mounted to a fixedaircraft surface, while the connecting member 18 is connected to amember to be actuated. There may be pivotal movement between theconnecting member 18 and the member to be actuated. However, theconnecting member is held against rotation about the axis of theballscrew whereby the ballscrew 10 cannot rotate, but can move linearly.

The ballscrew 12 may be caused to rotate upon operation of one or bothof a pair of power drive units 20 and 22. Each of these power driveunits, as shown for power drive unit 20, has an axial piston motor 24and a releasable brake 26 for controlling a drive shaft having a gear 28thereon which drive separate inputs to a speed-summing gearbox. Examplesof usable power drive units are disclosed in the Flippo U.S. Pat. No.4,191,094 and the Aldrich U.S. Pat. No. 4,210,066. The gear 28 mesheswith a rotatable gear 30 carrying a sun gear 32 which meshes with aplanetary gear 34. The other driven gear drives a sun gear 36 whichmeshes with a planetary gear 38. The planetary gears 34 and 38 arerotatably mounted on a planetary carrier 40 and mesh with a fixed ringgear 42 whereby driving of the planetary gears 34 and 38 causes rotationof the planetary carrier 40. The planetary carrier is rotatably mountedwithin the housing by ball bearings 42 and is fixed to the ballnut 12.

The ballnut 12 has ball grooves 50 receiving annular rows of balls whichcoact with a helical groove 52 formed on the exterior of the ballscrew10.

With the foregoing structure, as the power drive units 20 and 22 areplaced into operation, the ballnut 12 is caused to rotate and, dependenton the direction of rotation thereof, the ballscrew 10 will be caused tolinearly translate in one direction or the other to transmit forcethrough the connecting member 18. The use of the speed-summing gearboxenables continued operation of the ballscrew actuator, even if one ofthe power drive units 20 and 22 should become inoperable.

The ballscrew actuator is shown in FIG. 1 in a fully-retracted positionwherein buttress-shaped elements 56 are in contact to prevent fullfacial engagement between planar surfaces to permit freedom of relativerotation between the ballnut 12 and the connector 18 when the ballscrewis moved outwardly from retracted position. There are similarbuttress-shaped elements 58 and 60 which coact to prevent facialengagement when the ballscrew 10 is fully extended. A member 62 fixed tothe ballscrew facilitates rotation thereof.

The ballscrew actuator is of a ballistic tolerant, dual load pathstructure by having the ballscrew 10 formed of inner and outer tubularmembers and with the inner tubular member formed of a ballistic tolerantcomposite material.

An outer tubular member 70 of the ballscrew 10 has the external helicalgroove 52 and may be formed of a hardened steel material to provide aprimary load path between the ballnut 12 and the connecting member 18.When the ballscrew actuator is subject to ballistic fire, a ballistichit can cause the outer tubular member to crack in a way that results infailure thereof. The inner tubular member 76 is associated with theouter tubular member and formed of a ballistic tolerant material whichcan survive being hit by a projectile passing through the outer tubularmember to provide a secondary load path by maintaining the outer tubularmember 70 in operative condition for coaction with the ballnut 12.

The inner tubular member 76 is associated with the outer tubular memberby bonding and a mechanical connection, as more particularly describedhereinafter. The tubular and mechanical joint members are inclose-fitting bonded relation whereby the inner tubular membercontributes to the energy absorption of the outer tubular member 70.

This association of the inner and outer tubular members is moreparticularly shown in FIG. 3. The inner and outer tubular members are inclose-fitting relation and adhesively bonded to each other. The innertubular member has upturned ends 80 and 82 which lock the memberstogether. To assure added reliability and transmission of torsionalloads, the members are also mechanically connected together, as by pins84 and 86 passing through aligned openings in both of the members.

Alternatively, the inner and outer tubular members can be mechanicallyconnected together at their ends by means pre-loaded in tension.

The inner tubular member is formed of a ballistic tolerant compositematerial which comprises resin-bonded fibers 90. In at least oneembodiment, the fibers are cross-woven, as seen in FIG. 3. The fiberselection and the formation thereof into a tubular member can beselected to meet the desired requirements of ballistic tolerance, asmore particularly described hereinafter.

The design of the inner tubular member 76 of the ballscrew can be variedto provide for different handling of a projectile, depending on the sizeand nature of the ballistic threat and the wall thickness of the steelouter tubular member 70. If a projectile which has cracked and passedthrough the steel outer tubular member 70 is to pass through the innertubular member 76, the composite material of the inner tubular membercan be cross-woven fiber glass rovings in a resin binder of epoxy orpolyester. With the cross-weaving as seen in FIG. 3, it will be seenthat a projectile can pass through the glass fibers, like a pencilthrough a screen door. The fibers will be damaged, but crack propagationcannot occur. This action is illustrated in FIG. 4 wherein a projectilehas a travel path as shown by the broken line arrow A and passes throughthe walls of both the outer tubular member 70 and the inner tubularmember 76. With such a ballistic hit, the inner tubular member retainsits structural integrity whereby, with its association with the outertubular member, the latter member is maintained in operative conditionto coact with the ballnut 12 for transmission of loads through thesecondary load path established by the inner tubular member.

Another design of the inner tubular member 76 would be to have the innertubular member act as a "catcher's mitt" wherein, as seen in FIG. 5, aprojectile 130 can pass through the outer tubular member 70 of steel,but the inner tubular member 76 will catch the projectile. Althoughthere will be some inward deformation of the inner tubular member, therewill not be a failure thereof. This composite material to achieve thisresult can be achieved by use of a fiber in a resin binder to provide acomposite having a very high strength-to-weight ratio and which acts asan energy absorber when sized to do so. An example of such composite isthe use of an aramid fiber with such fiber being an aromatic polyamidefiber of extremely high tensile strength and greater resistance toelongation than steel. One commercial product of this type is marketedunder the trade name KEVLAR. This material has been known for use inbullet-resistant structures. In one example of such structure, the innertubular member has a wall thickness of 0.7" and is formed of KEVLAR 29which is wound on a mandrel in layers, with one layer being 0.010" and,therefore, the material being laid up in 70 layers and, after winding,is heat-cured to form the rigid composite material.

FIG. 2 shows an alternate embodiment of ballscrew actuator havingtelescoping ballscrews driven by a torque-summing gearbox.

A pair of power drive units 100 and 102 each have an output shaftdriving a gear with the power drive unit 100 shown as having outputshaft 104 driving a gear 106. The two output gears mesh with a commongear 108 in a torque-summing gearbox which is associated with a firstballnut 110 associated with a first ballscrew, indicated generally at112. A second ballnut, indicated generally at 114, is connected forrotation with the first ballscrew 112 and coacts with a second innerballscrew, indicated generally at 116. The second inner ballscrew 116 isrotatably associated with a connecting member 118 which can be pivotallyconnected to a member to be actuated and which is held against rotationaxially of the ballscrew by its pivotal connection. The construction ofthis ballscrew actuator is more particularly shown and described in thecopending application of Duane Grimm et al entitled Jam TolerantRedundant Drive Ballscrew Actuator, Ser. No. 686,984, filed Dec. 27,1984, and reference may be made thereto for more details regarding thisstructure.

As seen in FIG. 2, the first ballscrew 112 has an outer tubular member120 and an inner tubular member 122 constructed as described inconnection with FIGS. 1 and 3. The inner ballscrew 116 has an outertubular member 124 and an inner tubular member 126, with the secondballscrew 116 also being constructed as described in connection withFIGS. 1 and 3. In the operation of the structure of FIG. 2, rotation ofthe ballnut 110 will cause action of the first ballscrew 112 as well asthe second ballnut 114 to cause extension of the ballscrews in anysequence, dependent upon the resistance encountered between the ballnutsand the ballscrews.

FIG. 6 diagrammatically illustrates a telescoping ballscrew of the typeshown in FIG. 2 and illustrates different reactions to a projectile hit.The inner ballscrew 116 has an outer tubular member 124 and an innertubular member 126, with the latter member being constructed of amaterial as described in connection with FIG. 5, but with a thinner wallto have a lesser total energy absorption. Thus, an "aligned" round,indicated by the arrow 140, which passes through the outer tubularmember 124 may pass entirely through the inner ballscrew 116 while a"tumbled" round, indicated at 142 having lesser energy will be caught bythe inner tubular member 126 acting as the "catcher's mitt" previouslyreferred to. This structure can either be the inner ballscrew of atelescoping ballscrew actuator, as illustrated in FIG. 6, or can be thestructure of a ballscrew utilizing a single ballscrew as in theembodiment of FIG. 1.

FIG. 6 additionally illustrates a case different from those described inconnection with FIGS. 4 and 5 as well as previously described inconnection with the inner tubular member of FIG. 6. The outer ballscrew112 has the inner tubular member 122 thereof formed similarly to thestructure of FIG. 5 to function as a "catcher's mitt" and the innerballscrew 116 can function as previously described. In thisconfiguration, the outer ballscrew 112 serves as protection for theinner ballscrew when the actuator is retracted. This prevents thepotential of a jam, due to a lodged projectile unable to penetratethrough the eight wall thicknesses of the telescoping ballscrew andallows the inner ballscrew 124 to operate. The outer ballscrew 112 canstill hold loads but is unlikely to translate through the ballnut 114when it is damaged.

The dual load path ballscrew actuator with the use of ballistic tolerantmaterial provides increased reliability in aircraft subject to ballisticfire by increasing the probability of continued operation of primaryflight control components and thus successful return from the mission.Although it is known to have redundant load paths in a ballscrew, therehas been no assurance of failsafe operation upon the occurrence of aballistic hit which will crack the steel outer member of the ballscrewand comes into contact with the inner tubular member of conventionalconstruction providing the secondary load path. The use of ballistictolerant materials for the inner tubular member results in continuedstructural integrity of the inner tubular member to provide a secondaryload path and provide for continued coaction of the ballscrew with theballnut for linear translation of the ballscrew.

A modification of the invention to facilitate catching of a projectileby the inner tubular member is shown in the embodiment of FIG. 7. Thestructure which is the same as shown in FIG. 3 is given the samereference numeral with a prime affixed thereto. More particularly, theinner tubular 76' which, in this instance, is of the catcher's mitttype, for example, multi-layer KEVLAR, has a surrounding shell 130 ofceramic material which is bonded to or co-cured with with inner tubularmember 76'. This ceramic material has the hardness to blunt or shatter aprojectile and by removal of energy from the projectile allows the innertubular member to catch the projectile or projectile fragments. Anexample of such a ceramic material is partially-stabilized zirconia(PSZ).

FIGS. 8 to 12 illustrate another embodiment of the invention havingstructure to prevent axial separation of a circumferentially-crackedouter tubular member. This permits the balls of the ballnut to continuerolling on the threads of the ballscrew and avoids creation of a gapthat might cause the balls to jam. The inner tubular member of theballscrew is formed of composite material to provide the necessaryballistic tolerance and to enable pre-loading in tension as well astransmission of torsional loads. Pre-loading in tension precludes theaxial separation of an outer tubular member having a circumferentialcrack.

More particularly, the embodiment shown in FIGS. 8 to 12 has telescopingballscrews driven by a torque-summing gearbox in generally the samemanner as shown in the embodiment of FIG. 2.

One of a pair of power drive units is shown at 200 and has a hydraulicmotor as well as a hydraulically-controlled brake and an output whichdrives a gear 206 which drives through gears 208 and 210 on a commonshaft, a gear 212, which is also gearingly connected to a second powerdrive unit (not shown).

The gear 212 is associated with a first ballnut, indicated generally at214, rotatably supported in a housing 216 by bearings 218. The ballnut214 is rotatably associated with a first ballscrew, indicated generallyat 220, by a series of balls 222. A second ballnut, indicated generallyat 230, is connected for rotation with the first ballscrew 220 and hasballs 262 coacting with a second inner ballscrew, indicated generally at232.

The second inner ballscrew 232 is in longitudinally-fixed relation witha housing 234 which can have a connecting member associated therewith,such as the connecting member 118, shown in FIG. 2. This connectingmember holds the housing 234 against rotation, while there islongitudinal movement thereof. The housing 234 is rotatably associatedwith the second inner ballscrew 232 through ball bearings 236.

Operation of the pair of power drive units, including the power driveunit 200, shown in FIG. 8, results in rotation of the ballnut 214 withresulting linear translation of the first ballscrew 220 and the secondballscrew 232 in the manner generally known in the art.

The housing 234 mounts a brake and auxiliary drive structure not formingpart of the invention. The auxiliary drive structure includes a motor240, selectively operable to rotate a shaft 242 which, through a gearmesh at 244, can rotate the inner ballscrew 232. Selectively operablebrake structure can hold the shaft 242 against rotation and a secondbrake can be rendered operable, through a connection to the innerballscrew, by means of a gear mesh 246 and a sleeve 248. When the brakesare active, the inner ballscrew is held against rotation, wherebyrotation of the second ballnut 230 during normal operation will resultin linear movement of the inner ballscrew 232. When there is a failureof the primary drive including power drive unit 200 and actually a jamof the drive through the torque-summing gear 212, the brakes can bereleased and the motor 240 operated to cause rotation of the innerballscrew 232 relative to the second ballnut 230 which is held againstrotation due to the jam, to obtain linear movement of the innerballscrew 232. This mode of operation is particularly described in theGrimm et al application, previously identified herein.

The structure shown in FIG. 8 is shown on a larger scale in FIGS. 9 and10 and more particularly illustrate the construction of the ballscrewactuator which renders it ballistically tolerant and which provides forpre-loading.

Each of the ballscrews 220 and 232 has inner and outer tubular members.The ballscrew 220 has an outer tubular member 250 with an externalhelical groove 252 for coaction with the balls 222 and an inner tubularmember 254. The ballscrew 232 has an outer tubular member 256 and aninner tubular member 258, with the outer tubular member having anexternal helical groove 260 for coaction with the balls 262 of theballnut 230. The outer tubular members of the ballscrews may be formedof a hardened steel material to provide the primary load paths and, whenthe ballscrew actuator is subject to ballistic fire, a ballistic hit cancause an outer tubular member to crack in a way that results in failurethereof. The inner tubular member of each of the ballscrews functions tomake the ballscrew ballistic tolerant, as in the other embodiments and,more particularly, provides a pre-loading in tension on the innertubular members whereby, if the outer tubular member of either ballscrewshould have a circumferential crack, this crack is not permitted toexpand into a gap which might cause a jamming of balls of the ballnutsin the gap.

Referring particularly to the ballscrew 232, the inner tubular member258 is formed of a composite material of wound fibers in a binder, suchas a resin. The winding of the fibers provides for a ballistic tolerantcharacteristic as well as for loading in tension and mechanicalinterlocking to assure proper tensional loading and transmission oftorque. The inner tubular member is wound upon a retained mandrel 270which can be formed of graphite and which extends between a pair ofcylindrical members 272 and 274. In forming the inner tubular member,the mandrel 270 and associated cylindrical members 272 and 274 can bemounted in a winding machine and windings applied thereto. There are twotypes of windings applied to the mandrel and the cylindrical members.One winding provides for ballistic tolerance and torque transmission andis a winding of fibers 280 at a winding angle of approximately 45°. Thisfiber can be the previously described KEVLAR fibers. The other windingsare of fibers having substantial tensile strength and low creep andwhich are wound at a small angle relative to the longitudinal axis ofthe inner tubular member, in order to enable the pre-loading of theinner tubular member. As an example, these latter fibers can be wound atan angle of approximately 12° and can be graphite fibers. These fibersare identified at 282 in FIG. 12 and, after winding of the desiredthickness of the two types of fibers, the fibers can be bonded togetherwith a suitable bonding material, such as a resin, to form the compositematerial indicated generally at 284.

The cylindrical member 274 is dome-shaped, with the dome thereof ineffect forming a shoulder to provide for interlocking of the cylindricalmember with the composite material 284, as seen in FIG. 9, to providefor axial locking therebetween. Additionally, the cylindrical member 274is provided with a series of flats 288, as seen in FIG. 11, whichrotationally locks the composite to the cylindrical member for torquetransmission. Similarly, the cylindrical member 272 has a shoulder 290for axial locking and a series of flats 292 for torsional locking. Theinner tubular member can be placed in tension by structure including atubular extension 300 of the cylindrical member provided at an end witha peripheral flange 302 which can abut an end of the outer tubularmember 256 and tensioning means associated with the cylindrical member274. This tensioning means is seen particularly in FIGS. 8 and 9 andincludes a threaded stem 306 integral with the cylindrical member 274which is threadably associated with a nut 308 which coacts with the gear310 forming part of the gear meshes 244 and 246 and which is physicallylocated against shoulders at an end of the outer tubular member 256 ofthe ballscrew 232 and splined thereto Rotation of the nut 308 canprovide the desired tensioning force for pre-loading of the innertubular member. A structure within the housing 234 includes a cup-shapedsleeve 320 and a nut 322 threaded onto the stem 306.

The inner tubular member 254 of the ballscrew 220 is formed of the samewindings as previously described for the ballscrew 232 with theformation thereof on a retained graphite mandrel which is associatedwith a first cylindrical member 330 defined by part of the ballnut 230and which has a peripheral groove defining a locking shoulder 332 andflats, such as the flats 288 shown in FIG. 11. At the opposite end ofthe inner tubular member 254, the composite material is wound on acylindrical member 336 having a peripheral groove for an interfit asshown at 338 and an annular locking ring 340 provides an axial lock at342 and with there being serrations at 344 between the inner tubularmember and the locking ring 340 for transmitting torque.

The inner tubular member 254 is placed in tension by a pre-load nut 350(FIG. 9) which abuts against an end of the first ballnut 214 and isthreadably engaged at 352 to an end of the second ballnut 230 which hasthe cylindrical member 330 structurally associated therewith. Inassembly of the ballscrew 220, a locking ring 340 is first associatedwith the composite material of the inner tubular member and theperipheral groove 338. The outer tubular member 250 is then axiallyadvanced to a position adjacent the locking ring 340 followed byplacement of the pre-load nut 350 and rotation thereof to impartpre-load to the inner tubular member 254.

With the construction shown in FIGS. 8 to 12, the inner tubular membersof both ballscrews are preloaded whereby, if there is a circumferentialcrack in the outer tubular member, the crack is not permitted to expand.Additionally, the structure of the inner tubular members is ballistictolerant because of the windings of a ballistic tolerant material, suchas KEVLAR, wound at an angle to provide for transmission of torque,while the other windings at a relatively small angle enable theeffective pre-loading.

We claim:
 1. A ballistic tolerant dual load path ballscrew actuatorhaving an outer tubular member of metal subject to cracking when hit bya projectile and having an external helical groove for coaction with aballnut and a second tubular member positioned within and having alength generally coextensive with said outer tubular member, saidmembers being secured to each other adjacent their ends whereby thesecond member may react to loads applied to the ballscrew in the eventthe outer tubular member fails, the improvement wherein said secondtubular member is formed of a rigid ballistic tolerant nonmetallicmaterial which maintains its structural integrity without crackingthereof when hit by a projectile whereby the second tubular member willprovide a secondary load path after the outer tubular member has crackedand prevent lengthwise elongation of the outer tubular member and avoidincrease in the size of the crack in the outer tubular member whichcould cause the ballnut to malfunction, and said second tubular memberhaving a surrounding shell of ceramic material bonded thereto.
 2. Aballistic tolerant dual load path ballscrew actuator having a ballscrewwith an outer tubular member of metal subject to cracking when hit by aprojectile and having an external helical groove for coaction with aballnut and a second tubular member positioned within and having alength generally coextensive with said outer tubular member, saidmembers being secured to each other adjacent their ends whereby thesecond member may react to loads applied to the ballscrew in the eventthe outer tubular member fails, the improvement wherein said secondtubular member is formed of a rigid ballistic tolerant nonmetallicmaterial which maintains its structural integrity without crackingthereof when hit by a projectile whereby the second tubular member willprovide a secondary load path after the outer tubular member has crackedand prevent lengthwise elongation of the outer tubular member and avoidincrease in the size of the crack in the outer tubular member whichcould cause the ballnut to malfunction, said ballscrew being the innerballscrew of a pair of telescoping ballscrews, and a ceramic shellaffixed to the exterior of said composite of nonmetallic materials.
 3. Aballistic tolerant dual load path ballscrew actuator comprising, ahousing, a power drive unit mounted on said housing, a ballnut having aplurality of balls rotatably mounted on said housing and a dual loadpath ballscrew associated with said ballnut, said ballscrew having anouter tubular member of metal with an external helical ball groovereceiving said plurality of balls providing a primary load path and aninner tubular member positioned within the outer tubular member toprovide a secondary load path and with the ends of said members securedto each other, and said inner tubular member being formed of an aramidfiber composite of nonmetallic materials which will have an improvedcapability of maintaining structural integrity when subject to aballistic hit passing through and cracking said outer tubular memberwhereby the actuator may continue to function by enabling the balls tocontinue to roll over the cracked outer tubular member, and a ceramicshell affixed to the exterior of said aramid fiber composite.
 4. Aballistic tolerant dual load path ballscrew having an outer tubularmember with an external helical groove for coaction with a ballnut andan inner tubular member closely fitted within the outer tubular member,said inner tubular member being formed of a composite of materials, andmeans connecting said tubular members together adjacent their ends fortransmission of torsional loads and with the inner tubular memberpre-loaded in tension.
 5. A ballistic tolerant dual load path ballscrewas defined in claim 4 wherein said composite of materials includes twodistinct layers of different fibers wound at different winding anglesand a resin binder.
 6. A ballistic tolerant dual load path ballscrew asdefined in claim 5 wherein one of said fibers has strength in tensionand is wound at a small winding angle to enable said pre-loading intension.
 7. A ballistic tolerant dual load path ballscrew as defined inclaim 6 wherein said one fiber is a graphite fiber.
 8. A ballistictolerant dual load path ballscrew as defined in claim 5 wherein one ofsaid layers is formed of a fiber wound at a relatively large windingangle and said layer is a ballistic tolerant layer.
 9. A ballistictolerant dual load path ballscrew as defined in claim said fiber is anaromatic polyamide fiber.
 10. A ballistic tolerant dual load pathballscrew as defined in claim 4 wherein said inner tubular member has afirst lay of low creep fiber windings bonded in a resin with the fiberswound at a small winding angle to enable said pre-loading in tension,and a second layer of fiber windings bonded in a resin and bonded to thefirst layer with the fibers wound at a greater winding angle and of amaterial to render the inner tubular member ballistic tolerant.
 11. Aballistic tolerant dual load path ballscrew as defined in claim 4wherein said means connecting said tubular members together includes apair of cylindrical members at the ends of the inner tubular member andeach having a shoulder and a part of the outer periphery formed withflats, said inner tubular member being formed of wound fibers which arewound about said cylindrical members to surround the flats to provide atorsion-transmitting connection and which engage behind said shouldersto enable placing said inner tubular member in tension, and saidconnecting means further includes members operatively connected to theouter tubular member engaging said cylindrical members and operable toapply tension to the inner tubular member.
 12. A ballistic tolerant dualload path ballscrew as defined in claim 11 wherein one of saidcylindrical members has a dome-shaped portion defining said shouldersand a threaded stem for coaction with one of said members.
 13. Aballistic tolerant dual load path ballscrew as defined in claim 4wherein said means connecting said tubular members together includes apair of cylindrical members associated with the outer tubular member andeach having a shoulder, said inner tubular member being formed of woundfibers which are wound to form a pair of shoulders at the ends thereoffor interlocking coaction one with each of said cylindrical members,means for imparting a separating force to said cylindrical members toplace the inner tubular member in tension for pre-loading thereof.
 14. Aballistic tolerant dual load path ballscrew as defined in claim 13wherein said inner tubular member is wound onto said cylindrical membersto place said shoulders in interlocking relation.
 15. A ballistictolerant dual load path ballscrew as defined in claim 13 includinginterfitting means to rotationally lock the inner and outer tubularmembers together to provide a torsion-transmitting connection.